During recent years Al-Li alloys have become metals of great interest in the aerospace industry due to their extreme light weight. However, in order for these metals to achieve necessary strength, they must be heat treated. Typically, these treatments include solution heat treatment, homogenization, or annealing. During such heat treatment lithium atoms in the vicinity of the surface combine with oxygen to form oxides. In time there is an increased lithium atom vacancy concentration in the interior of the alloy material as lithium atoms diffuse toward the surface forming a gradient. The result is a net motion of vacancies which is a vector quantity having magnitude and direction referred to as a vacancy flux. In vector terms, the vacancy flux added to the aluminum atom flux is equal to the oppositely directed flux of lithium atoms. The existence of the vacancy flux physically creates agglomerated vacancies which resemble internal microscopic pores near the surface where the lithium has been lost. The absence of lithium and the presence of pores diminishes the strength of the alloy.
Certain prior art approaches have attempted to create a protective atmosphere during heat treatment. For example, in published U.K. patent application 2,137,666 A, a carbon dioxide atmosphere having a controlled water vapor partial pressure constitutes the heat treatment atmosphere. A protective atmosphere of this type is considered to decrease the attack rate on lithium atoms due to oxidation. However, as shown in the figure, with a "wet" carbon dioxide atmosphere, a substantial lithium loss still occurs for some distance adjacent the surface. It would be highly desirable to minimize the "rising edge" portion of the plot shown in the figure.